Darwin described at least two types of movements in plants. In the second lecture, we talked about phototropism, plants bending towards the light, and here we've learned about gravitropism. Plants bending either towards gravity or away from gravity. But we haven't talked about what's causing the bending. And if you remember, Darwin concluded from his phototropism experiments that there was a signal moving from the tip to the base. Or as he wrote in his book, we must therefore conclude that when seedlings are freely exposed to lateral light, some influence is transmitted from the upper to the lower part, causing the latter to bend, an influence. What is this influence? What is this signal? Well, Darwin didn't know, which is why he called it an influence. But in the 140 years, 130 years since Darwin did his experiments. We know what this influence is. So how is this influence discovered? So, the first experiments that we studied, we're now we're going back to talk a bit about phototropism because this will help us understand gravitropism. Where Darwin's experiments where we covered the tip of the plant with the light proof cap and we saw that it didn't bent. And that's where we came to his conclusion about the influence. Darwin's results were the basis for further experiments at the beginning of the 20th century by the Dutch scientist Peter Boysen-Jensen. And what, Boysen-Jensen did, was he cut off the tip of the seedling. And then put it back on the plant. Now when he cut off the tip and put it back on the plant, the plant still had the ability to bend towards the light. So he then did the following experiment. He cut off the tip. And then put it back on the plant, but before he put it back, you put a slice of gelatin on the stalk, and then put the plant tip back on it. In the second plant, he put a piece of mica, a piece of glass, between the stalk and the tip. In the first case, when the tip and the stalk are separated by a piece of gelatin, the plant still retained the ability to bend, whereas when it was separated, the tip was separated from stark by piece of glass, it lost the ability to bend to light. It also lost the ability to respond to light. It lost the ability to do phototropism. So what's the conclusion from this experiment? The conclusion is that the tip of the plant is generating a soluble signal that is somehow traveling down the plant, influencing it to bend. So Darwin's influnce is a soluable signal. But the question then of course is what is the soluble signal? And the soluble signal was isolated in 1926 by another scientist named Went. Now his hypothesis was that the gelatin or what we actually call agar contains a chemical produced by the tip which stimulates growth further down the stock. So to test the hypothesis, he did the following experiment. He took the tip, and then put it isolated on top of the agar. Let it stand for a little bit, and then placed the agar directly on the stalk of the plant, a plant that had had its tip cut off, a decapitated plant, but without the tip itself. What he saw was that when he put the agar squarely on top of the stalk, the stalk then resumed growing up. Again this is a stalk that itself does not have a tip. But if you just put the agar itself that had never, add a slice of agar that had never exposed to the tip of the plant. There was no, there was no influence on the stalk, it did not grow. What this is showing is that it wasn't the agar itself that contained a chemical that was causing the plant to grow. But rather that the agar that had been exposed to the tip. Contained some type of chemical which was causing the plant, the stalk, to elongate. Then if he put the agar on only half the stalk, so for example, let's say this is the stalk and I'm only putting the agar here on one side. What would happen was that the plant would then bend towards the other direction. If I were to put the agar again, on this side, then the plant would bend to the opposite direction. In other words, somehow or another, the agar was causing the plant to elongate, and if it was only on one side of the stalk, only one side is elongating. And that's sort of like what was happening in phototropism. He isolated from this agar a chemical, which was then called auxin, which is taken from the Greek word to increase, in other word, to increase its length. So auxin is the first plant hormone. Its a phytohormone to be isolated. And one of the roles of auxin is to stimulate growth.